Conventionally, a system such as an ETC, a VICS and a GPS has been widespread, and therefore an antenna used for such a system is commonly mounted in a vehicle. The antenna is typically mounted in or in the vicinity of an instrument panel provided in the front part of a vehicle interior, so as to favorably receive a radio wave from the outside of the vehicle.
On the other hand, well-known is an antenna apparatus which has a radome for enclosing an antenna thereof so as to, for example, protect the antenna (for example, Patent Document 1). The radome is typically made of synthetic resin material having a uniform thickness, or the like, and positioned so as to enclose the entire radiating surface of an antenna element.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-273639